In recent years, sophisticated law enforcement agencies have recognized the benefit of marking or tagging various types of bulk goods with tiny identifier particles. Such identifier particles or "microparticles" are designed to be imperceptible to the naked and untrained eye, but permit the bulk goods to be positively identified as to owner, manufacturer, lot number, etc. at some later date. If such tagged goods or items are diluted, adulterated, stolen, or illicitly-used, this fact can be readily determined and the guilty party more easily prosecuted. The most notable use of such microparticles has been in the manufacture of explosives. The microparticles are designed to survive detonation and can be retrieved from debris surrounding the blast.
U.S. Pat. Nos. 3,772,200, 3,772,099 and 3,861,886 describe various types of microparticles designed for tagging bulk goods such as explosives. In general, rather sophisticated instrumentation such as a microprobe analyzer or spectroscope is required to decipher the code which is built into these particles. In other words, the code is not visually readable, and a particle must be removed from the tagged substance and subjected to instrumentational analysis in order to decipher the code, often at a site remote from the area where the particles are retrieved.
A significant improvement in the aforementioned microparticles is disclosed in U.S. Pat. No. 4,053,433. This improvement relates to particles encoded with an orderly sequential arrangement of visually-distinguishable colored segments. The code of these microparticles can be read visually at the site of retrieval with the aid of a simple microscope or magnifying glass. The advent of such color-coded microparticles significantly expanded the use of microparticles because sophisticated readout equipment was eliminated. Improved color-coded microparticles (over those disclosed in U.S. Pat. No. 4,053,433) are described in German Offenlegungsschrift No. 26 51 528. These microparticles are irregularly-shaped and economically prepared by random comminution of brittle laminates formed of colored-layers laid down in a predetermined sequence to constitute the code. Because the code of these microparticles could be read visually with a simple hand-held magnifying glass, it was no longer necessary to separate the particles from the tagged goods, and the microparticles could be used to label the surface of solid objects, as by permanently attaching the particles to the surface with a clear lacquer. In this way, items of personal property such as hand tools, maintenance equipment, vehicles, machinery, etc. could be easily labeled by painting or spraying a transparent liquid composition containing the microparticles onto a small surface area of the item. Such a tagging method works well for retrospective identification purposes once a tagged object has been located and the area where the microparticles have been deposited is determined. However, because the microparticles are substantially indiscernable under normal viewing conditions, a problem was encountered in determining quickly and easily whether or not an object was tagged, particularly if one wished to make such a determination from even a slight distance from the object.
Accordingly, a need has developed for a transparent coating for adhering microparticles to a surface which is indiscernable by normal visual inspection, yet quickly and easily discernable by one who is trained to detect its presence on the object. The compositions of the present invention uniquely meet the aforementioned need by providing coatings containing spotting particles which render the tagged surfaces readily identifiable when a beam of light is focused on the object.